Recently, as energy problems, or global environmental problems become more serious, solar cells are attracting attention as alternative energy, replacing fossil fuel.
In this solar cell, light is applied to a photoelectric conversion part formed by a semiconductor junction or the like, to generate carriers (electrons and electron holes). Then, the generated carriers are extracted to an external circuit, thereby generating electric power. On the photoelectric conversion part of this solar cell, a collector electrode is provided in order to effectively extract the carriers generated by the photoelectric conversion part, to the external circuit.
Examples of the solar cell include, for example, a crystal silicon-based solar cell using a single crystal silicon substrate or a polycrystal silicon substrate. In this crystal silicon-based solar cell, a collector electrode made of thin metal is provided on a light-receiving surface.
Additionally, examples of the solar cell include a heterojunction solar cell having an amorphous silicon layer and a transparent electrode layer on a crystal silicon substrate. Also in the heterojunction solar cell, a collector electrode is provided on the transparent electrode layer.
Generally, the collector electrode of the above solar cell is often formed by printing a silver paste material in a predetermined pattern by a screen printing method. However, this method has a problem that a material cost for silver increases, while the process itself is simple. Additionally, in this method, a silver paste material containing a resin is used, and therefore there is a problem that the resistivity of the collector electrode is higher compared to a case of using a pure metal. Therefore, in the case of forming a collector electrode by using the silver paste material, it is necessary to print the silver paste material thick, in order to reduce the resistivity of the collector electrode. This causes a problem, in that manufacturing cost increases. When the printing thickness increases, the line width of the formed collector electrode also increases. Therefore, it is difficult to thin the electrode, and there is another problem that a shading loss due to the collector electrode increases.
As a method for solving theses problems, there is known a method for forming a collector electrode by a plating method. This plating method is generally more excellent than a printing method in material cost and cost in manufacturing (process cost).
For example, Patent Documents 1 to 3 each disclose a method for producing a solar cell in which a metal layer made of copper or the like is formed on a transparent electrode configuring a photoelectric conversion part by a plating method.
The method for producing a collector electrode disclosed in Patent Document 1 will be described. First, a resist material layer (insulating layer) having an opening corresponding to the shape of a collector electrode is formed on a transparent electrode layer of a photoelectric conversion part. Then, a metal layer is formed by electroplating in the resist opening of the transparent electrode layer. Thereafter, a collector electrode having a predetermined shape is formed by removing the resist.
Patent Document 3 discloses that the line width of a plating electrode layer is made to be not larger than the line width of a base electrode layer by forming a plating electrode layer by using a mask after the formation of the base electrode layer.
Additionally, Patent Document 3 discloses that a plating solution adhered to a substrate is cleaned and removed by water, an organic solvent, and the like after a plating step, in view of a problem that when a solar cell having the remaining plating solution is exposed to a high temperature and high humidity environment, a solar cell property is deteriorated.
In Patent Document 4, after a translucent insulating layer of silicon dioxide (Sift) or the like is provided on a transparent conductive film, a groove that passes through the translucent insulating layer is provided and the surface or the side surface of the transparent conductive film is exposed. Patent Document 4 discloses a method for forming a metal collector electrode so as to be electrically connected with an exposed part of the transparent conductive film.
Specifically, Patent Document 4 proposes a method for forming a metal seed in the exposed part of the transparent conductive film by a light plating method or the like, and forming a metal collector electrode by electroplating with this metal seed as a starting point.
This method described in Patent Document 4 does not need the use of a resist unlike Patent Document 1. Therefore, the method described in Patent Document 4 has an advantage over the method described in Patent Document 1 in material cost and process cost. Additionally, in the method described in Patent Document 4, a low-resistance metal seed is provided, so that it is possible to reduce contact resistance between the transparent conductive film and the metal collector electrode.
In Patent Document 5, unevenness of a conductive seed is enlarged. Consequently, when an insulating layer is formed, the whole surface of a part except the conductive seed of a photoelectric conversion part is covered, thereby forming a discontinuous opening on the conductive seed. Patent Document 5 describes the formation of a plating layer through this opening.
In addition to the above, there are Patent Documents 6 and 7 as a prior art related to the present invention.